In an internal combustion engine, a time lag occurs between an acceleration operation, for example, and the time at which the amount of air taken into the engine cylinders actually increases. When the engine is running normally and fuel is injected through a fuel injector in an amount corresponding to a target fuel injection amount, which is calculated from an intake air flow rate detected by an air flow meter provided upstream of an intake throttle, and a target air-fuel ratio, the air-fuel mixture in the engine reaches the stoichiometric air-fuel ratio. However, during transitional operating periods of the engine, such as acceleration and deceleration, the air-fuel mixture in the cylinder deviates temporarily from the stoichiometric air-fuel ratio due to the delay in change of the intake air amount.
JP01-305144A, published by the Japan Patent Office in 1989, proposes a fuel injection amount calculation method for ensuring that the air-fuel mixture supplied to a cylinder matches a target air-fuel ratio even during transitional operating periods of an internal combustion engine for a vehicle. More specifically, an intake air amount per combustion cycle of the cylinder is calculated from a detected flow rate, detected by an air flow meter, and a target fuel injection amount corresponding to the intake air amount of the cylinder is calculated from this intake air amount and the stoichiometric air-fuel ratio. Moreover, in this prior art, the target fuel injection amount is applied at a timing advanced by ten milliseconds from the closing of an intake valve.